1. Field of the Invention
The present invention relates to fabrication processes for accelerometers and other force sensing devices, and more particularly to a fabrication process for producing a capacitive pendulous MEMS accelerometer with a symmetric plate area having an asymmetric plate mass.
2. Description of Related Art
High performance accelerometers with near micro-gravity resolution, high sensitivity, high linearity, and low bias drift are needed for a wide variety of applications, especially aerospace applications such as inertial navigation systems, guidance systems, and air data measurement systems. The resolution of high-performance accelerometers has been limited by thermomechanical Brownian noise of the sensor, which is dictated by the damping coefficient and the mass of the structure, as well as by the readout electronics.
Fabrication technology plays a critical role in ensuring that large mass, large capacitance, and small damping are simultaneously obtained, and that micro-gravity resolution is achieved. Previously, a number of high performance silicon accelerometers have been reported. These devices utilize a large proof mass in conjunction with capacitive, resonant, or tunneling current sensing schemes to achieve high sensitivity. Among all these, silicon capacitive accelerometers have several advantages that make them very attractive for numerous applications ranging from low cost, large volume automotive accelerometers to high precision inertial grade micro-gravity devices. Silicon capacitive accelerometers have high sensitivity, good direct current response and noise performance, low drift, low temperature sensitivity, low power dissipation, and a simple structure.
Capacitive accelerometers are typically vertical and lateral structures. Some designs use a see-saw structure, with a proof mass such as a flat plate suspended by torsional beams. The structure is typically asymmetrically shaped so that one side has greater mass than the other, resulting in a center of mass that is offset from the axis of the torsion bars. When an acceleration force produces a moment about the torsion bar axis, the plate is free to rotate, constrained only by the spring constant of the torsion bars.
The sensitivity of these types of accelerometers is defined as the ratio of deflection to acceleration. The mass of the plate, the distance from the center of mass to the torsion bar axis, and the torsion bar stiffness determine sensitivity. To increase the offset of the center of mass, the plate structure is designed to have an asymmetric shape. For example, one side of the plate may have a width that is larger than the other side of the plate, or one side of the plate may have a greater length than the other side. However, increasing the center mass offset by the asymmetric shaping methods mentioned above may result in an increase in total mass of the plate, which leads to reduced resonant frequency and decreased sensitivity. Increasing the center mass offset by asymmetric shaping may also result in a sacrifice of some of the dynamic g-range, which is defined by the separation distance between a stationary sensing element and the pendulous acceleration sensing plate. Another method for increasing center mass offset involves lengthening a portion of the pendulous sensing plate. The center mass offset is proportional to the length of the extended portion of the plate. However, extending one side of the plate may lead to unbalanced gas damping, which results in performance degradation. Gas damping can be balanced by perforating portions of the extended plate. However, such perforations also reduce the center mass offset and so reduces the sensitivity. Additionally, extending one side of the plate may result in an increase of the overall chip size.
Other conventional structures have utilized a deeper gap underneath the extended plate portion to increase the maximum angle of rotation while maintaining balanced gas damping. Such a structure may increase the dynamic g-range to some extent. However, the extended portion of the plate increases the dimension of the overall chip size, leads to unbalanced gas damping, and reduces the resonant frequency of the rotational structure, which again results in a decrease in the performance of the accelerometer.
An asymmetric pendulous MEMS accelerometer was disclosed in U.S. patent application Ser. No. 11/978,090, filed in the U.S. Patent and Trademark Office on Oct. 26, 2007, the disclosure of which is herein incorporated by reference. A pendulous accelerometer having a hollow plate structure exhibits higher sensitivity than that of a pendulous accelerometer having a solid plate structure of the same dimensions and the same capacitance. However, fabricating a pendulous accelerometer with such a hollow plate structure can present challenges.
Accordingly, there is a need for an economical and efficient process for fabricating a capacitive pendulous MEMS accelerometer with a symmetric plate area having an asymmetric plate mass that includes a hollow plate structure.